Catapults are really useless now.
My proposal to them would be a slight increase in distance, hp improved maybe to 200 or 250, and most importantly, their speed, it is slow for everything, to move, to mount / disassemble and mainly to shoot, I would like to sail More agile, especially at attack speed.
I would like to talk about the darts catapults too. These I have never come to be used, are totally inute in all alphas.
Needs a total refinement, especially in its accuracy and damage, and one thing that would be very cool and realistic, the dart could cross more than one unit in line.